The present invention relates to a semiconductor device such as a semiconductor integrated circuit, a method of processing a copper film to be used for wirings for a semiconductor device such as a semiconductor integrated circuit and a method of producing a wiring structure for such a semiconductor device.
Today, copper or a material essentially consisting of copper is used for wirings for a semiconductor device such as a semiconductor integrated circuit. In such a case, copper atoms are diffused into an insulating film to enter the semiconductor substrate, which may result in malfunction of transistors. To prevent it, a barrier metal layer of a titanium nitride, tantalum nitride, tungsten nitride or the like is often formed around the copper film to isolate the copper film from the insulating film.
As shown in FIG. 13A which is a cross-sectional view showing the structure of a semiconductor substrate, wirings formed on a semiconductor substrate are normally buried in wiring grooves in an insulating film. An inter-layer insulating film 101 is formed of silicon oxide on a semiconductor substrate 100 of silicon or the like where a semiconductor element (not shown) such as an integrated circuit is formed, and wiring grooves are formed in the surface of this inter-layer insulating film 101. A barrier metal layer 102 of TiN, TaN or the like is formed on the side wall of each wiring groove and a copper film 103 or an alloy film essentially consisting of copper is buried in the region surrounded by the barrier metal layer 102.
Apparently, the conventional wiring may cause copper in the wiring portion to be diffused into the inter-layer insulating film 101 from a portion where there is no barrier metal, i.e., from above, thereby adversely affecting the semiconductor element formed on the semiconductor substrate 100. As the surfaces of the inter-layer insulating film 101 and the copper film 103 buried therein are planarized by CMP or the like, the pattern edges may not be detected at the time of implementing lithography, which may lead to deviation in the wiring pattern.
Those shortcomings can be dealt with by setting back the wiring portion or the copper film 103 from the surface of the inter-layer insulating film by etching (FIG. 13B). Making such a shape can provide accurate pattern alignment.
There may be a case where a barrier metal layer 104 may be buried in the recessed portion to cover the copper film 103 as shown in FIG. 13C. This structure has an advantage of suppressing the diffusion of copper from above the wiring. Further, forming the cap layer of a conductive material prevents copper from being placed in the etching environment at the time a via wiring (contact wiring) to the over-lying wiring is formed. This can reduce the possible occurrence of corrosion or etching-originated wiring disconnection.
The above-described conventional wiring structures are formed by two methods: wet etching and dry etching. Dry etching includes anisotropic etching called RIE (Reactive Ion Etching) and isotropic etching called CDE (Chemical Dry Etching), both of which can etch copper. Those dry etching processes often use CF-based gas as the source gas, which adversely affect the environment of the earth. Further, because a by-product is deposited after etching, a wet process for removing the by-product is often performed so that dry etching is disadvantageous over wet process in the number of steps as well as the cost.
In this aspect, attention is being paid to wet etching. Copper is hardly dissolved in a weak acid which has a weak oxidizing property, such as hydrochloric acid, hydrofluoric acid, dilute sulfuric acid, acetic acid, hydrocyanic acid, or the like, however it is etched by an acidic chemical solution which has an oxidizing property. Specifically, such acidic chemical solutions include thermal concentrated sulfuric acid, nitric acid, nitrous acid, phosphoric acid and the like. Copper is also soluble in an acid solution which is formed by mixing acid with aqueous hydrogen peroxide, ozone or oxygen, such as hydrochloric acid+aqueous hydrogen peroxide, hydrochloric acid+aqueous ozone or hydrofluoric acid+aqueous hydrogen peroxide. Further, copper is also etched by a material which forms a soluble complex with copper, such as ammonia, a material having an amino group (ethylene diamine or the like), cyanide (KCN or the like). Etching is often accelerated by mixing aqueous hydrogen peroxide or the like with those materials to provide an oxidizing property.
Normally, a mixed solution of aqueous ammonia and aqueous hydrogen peroxide and a mixed solution of hydrochloric acid and aqueous hydrogen peroxide are used as cleaning chemical solutions and are respectively called SC1 and SC2. The concentrations of aqueous ammonia, hydrochloric acid and aqueous hydrogen peroxide available on the market are often about 20 to 35%, and SC1 and SC2 are usually mixed with pure water in the volume ratio of about 1:1:5 to 1:1:7. As copper is dipped into the solution mixed under the above condition, it is etched.
When copper is etched with SC1 or SC2 under the aforementioned condition, however, the copper surface having a metallic gloss becomes whitened and loses gloss. Etching copper with any of the aforementioned chemical solutions such as nitric acid and thermal concentrated sulfuric acid also causes the whitening phenomenon. This phenomenon becomes more prominent unless the temperature of the solution is raised. Whitening of the surface of copper occurs as etching roughens the copper surface. If this copper is used for wirings and the surface roughness becomes larger, a variation in film thickness or the surface scattering may increase the substantial resistance or result in contact failure to the over-lying wiring. It is therefore necessary to perform etching under such a condition as not to roughen the copper surface as much as possible.
Another way of etching copper is to oxidize a copper film and then remove the copper oxide using acid or the like. For example, Jpn. Pat. Appln. KOKAI Publication No. 2-306631 proposes a method of carrying out implantation of oxygen ions in a copper film and then annealing the resultant structure or subjecting the structure to an oxygen plasma treatment to form a copper oxide, and then etching the copper oxide with diluted sulfuric acid or acetic acid. In addition, Jpn. Pat. Appln. KOKAI Publication No. 10-233397 proposes a method of placing a copper film in the environment of oxygen (O2) or ozone (O3) in a diffusion furnace, an RTA furnace or an oven at room temperature or higher to thereby form a copper oxide film and then removing the copper oxide film by wet etching using diluted hydrochloric acid or diluted sulfuric acid, or dry etching or CMP. Those schemes however often cause copper to have a rough surface after etching. Particularly, this tendency becomes more noticeable as the temperature is increased to make the oxide film thicker.